1. Field of the Invention
The present invention relates to a method for preparing an anatase type titanium dioxide photocatalyst having a particle size of nano level and the photocatalyst prepared by the method and, more particularly, to a method for preparing a photocatalyst without a need of sintering process and a photocatalyst prepared by the said method.
2. Description of the Related Art
Studies on the photocatalysts, starting from the fields related to the conversion and storage of solar energy, have been vigorously made in connection with degradation of various organic compounds under UV radiation in the presence of a photocatalyst with a view to water purification wastewater treatment, deodorization in a refrigerator or vehicle, and so forth.
Those studies were initiated in 1839 when Becquerel generated voltage and current with a silver chloride electrode immersed in an electrolyte and connected with a pair electrode, and had a rapid progress since 1972 when Fujishima and Honda in Japan reported that a radiation on a TiO2 single crystal electrode decomposed water into hydrogen and oxygen.
Titanium dioxide is the most stable and popular photocatalyst of easiness in preparation that has ever been tested. In order to be used as a photocatalyst, titanium dioxide has to exhibit crystallinity as an anatase type. Thus, in a case where amorphous titanium dioxide is formed from a titanium-based starting material through hydrolysis and condensation polymerization, there is a necessity to carry out a heat treatment at high temperature, i.e., sintering process, in order to convert the amorphous titanium dioxide to an anatase type titanium dioxide. It is known that the sintering temperature is typically around 400xc2x0 C.
Among conventional methods using titanium dioxide as a photocatalyst, there can be distinguished as the most widely used methods: a first method of using an anatase type titanium dioxide in the powder form and a second one of using a thin film of the anatase type titanium dioxide formed on a specific support by a sol-gel method. Although the former is superior to the latter in the photocatalyst activity due to the relatively large surface area of the titanium dioxide prepared, the latter is more practicable in the aspect of stability of the titanium dioxide. Therefore, the actually used photocatalysts are prepared by the second method that involves formation of a titanium dioxide film on a support by means of a sol-gel method.
Now, a description will be given below to the conventional sol-gel method for preparing a photocatalyst in connection with FIG. 1.
FIG. 1 is a flow chart showing a process for preparing a titanium dioxide photocatalyst by the conventional sol-gel method.
The process includes the steps of: (a) preparing a TiO2 precipitate from an aqueous solution of a titanium-based starting material such as titanium alkoxide, TiCl4, TiOSO4, etc. by hydrolysis and condensation polymerization; (b) subjecting the precipitate to filtration to obtain a while amorphous titanium dioxide; (c) sintering the amorphous titanium dioxide at high temperature to obtain an anatase type titanium dioxide; (d) milling the resulting titanium dioxide to yield a powdery titanium dioxide; (e) dispersing the powder in a specific solvent to prepare an anatase type titanium dioxide solution; and (f) coating the solution on a support.
Such a conventional method for preparing a titanium dioxide photocatalyst is a multistage process that involves the steps of filtration, sintering, milling and dispersion, which may cause an increase in the unit production cost. The conventional method is also a troublesome in that there is a need of milling the anatase type titanium dioxide into very tiny particles, i.e., in the particle size of nano level, and dispersing the particles in a specific solvent prior to a coating step, in order to enhance the outer appearance of the finally coated photocatalyst or to increase the surface area of the photocatalyst to be in contact with the outside. With a large particle size of the anatase type titanium dioxide, an excessively large amount of the precipitate may be formed in the dispersion step and the resulting photocatalyst is inapplicable to a coating solution.
Also, the use of several additives for enhancing the coating characteristic and the hardness of the photocatalyst films may cause a problem to deteriorate the stability of the titanium dioxide disperse solution and result in formation of precipitates.
The most well-known anatase type titanium dioxide powder commercially available is P25 supplied by Degussa, which is now manufactured in a limited number of countries. There are also commercially available coating products containing a photocatalyst dispersed in water and ethanol, which are too expensive due to their complicate preparation process.
Accordingly, an object of the present invention, contrived to solve the above-stated problems with the prior art, is to provide a method for preparing an anatase type titanium dioxide photocatalyst having a particle size of nano level without a need of the sintering process at high temperature, and an anatase type titanium dioxide photocatalyst having a particle size of nano level prepared by the said method.
To achieve the object of the present invention, there is provided a method for preparing an anatase type titanium dioxide photocatalyst having a particle size of nano level including the steps of: adding a titanium-based starting material to a selected solvent; adding an acid or base catalyst to the resulting aqueous solution; subjecting the catalyst-containing aqueous solution to heat treatment at about 80xc2x120xc2x0 C. to activate peptization and thereby prepare an anatase type titanium dioxide sol solution; and coating the anatase type titanium dioxide sol solution on a support.